1. Field of the Invention
This invention relates to EL (electroluminescence) emitting touch switches utilizing EL in combination with a touch switch, and more particularly, to an EL emitting touch switch provided with a plurality of touch detection electrodes.
2. Description of the Related Art
Touch switches have been conventionally known which are provided with conductive electrodes for detecting the contact or proximity of part of a human body with or to the electrodes. In such touch switches, the electrode functions as a capacitance sensor, and the on/off operation of the touch switch is detected based on change in the impedance (electrostatic capacitance) of the electrode induced by the contact of a human body with the electrode.
There have also been known EL emitting touch switches in which the touch switch is combined with an organic or inorganic EL device for emitting light from a phosphor therein so that light may be emitted from the electrode upon detection of the contact of a human body with the electrode. This type of touch switch is disclosed, for example, in Unexamined Japanese Patent Publication No. H05-135654 (hereinafter referred to as Patent Document 1).
FIG. 3 is a schematic sectional view of an example of the body of the touch switch disclosed in Patent Document 1. As shown in FIG. 3, the body of the touch switch has a transparent, electrically insulating substrate 2, a transparent electrode 5 formed on a surface (upper surface) of the substrate 2, and a transparent insulating film 6 covering the upper surface of the electrode 5 and forming a touch surface.
Also, an EL layer 1 is formed on the reverse surface (lower surface) of the transparent insulating substrate 2. The EL layer 1 is formed by successively laminating, from the substrate side downward, a transparent electrode 1a, a phosphor layer 1b, an insulating layer 1c, and a back electrode 1d. When an alternating-current (AC) voltage is applied between the two electrodes 1a and 1d, the EL layer 1 emits light.
The emitted light propagates through the transparent electrode 1a, the transparent insulating substrate 2, the transparent touch detection electrode 5 and the transparent insulating film 6 to the outside of the touch switch, thus enabling the operator to easily locate the transparent insulating film 2 with the aid of the emitted light and touch the switch with his/her finger. When the transparent insulating film 6 is touched with the operator's finger, a touch decision circuit, not shown, connected to the transparent touch detection electrode 5 detects a change of the impedance or capacitance and judges that the transparent insulating film 6 is touched by the operator. Based on the judgment, the on-off control of a device connected to the touch decision circuit may be executed.
This touch switch essentially requires, however, a pair of electrodes 1a and 1d for driving the EL layer 1 to emit light, as well as the transparent touch detection electrode 5. It is therefore difficult to reduce the thickness of the touch switch, and also since the number of electrode layers is large, the cost increases.
As a solution to the problem, Japanese Patent No. 3284259 (hereinafter referred to as Patent Document 2) discloses an EL emitting touch switch in which the touch detection electrode and the EL driving electrode are unified into one common electrode, thereby making it possible to reduce the thickness of the touch switch.
FIG. 4 schematically shows the configuration of the EL emitting touch switch disclosed in Patent Document 2. As shown in FIG. 4, the switch body 10 of the touch switch includes an EL layer 1, which is a laminate having a phosphor layer 1b and an insulating layer 1c sandwiched between a transparent electrode (surface electrode) 1a and a back electrode 1d. The EL layer 1 is formed on an insulating substrate 3 and covered with a transparent insulating film 6. The transparent electrode 1a functions as both the touch detection electrode and the EL driving electrode.
Also, to the switch body 10 are connected a touch decision circuit 100 and an EL driver circuit 200 in parallel with each other. The touch decision circuit 100 detects change in potential of the transparent electrode 1a, to thereby detect the contact of part of the operator's body with the transparent insulating film 6. The EL driver circuit 200 outputs an AC voltage based on the potential of the transparent electrode 1a, to cause the EL layer 1 to emit light.
The switch body 10 is further provided with a drive control circuit 300, in addition to the touch decision circuit 100 and the EL driver circuit 200. The touch decision circuit 100 operates in accordance with a control signal from the drive control circuit 300. Also, the touch decision circuit 100 is so configured as to detect the contact of the operator's finger with the transparent insulating film 6 on the basis of change in the difference between the potential VA of the transparent electrode 1a and a ground potential VG. When the operator's touch on the transparent insulating film 6 is detected by the touch decision circuit 100, the drive control circuit 300 drives the EL driver circuit 200 to apply an AC electric field between the electrodes 1a and 1d, thereby controlling the emission of light from the EL layer 1.
It is conceivable that touch switches such as those described above are applied to a control panel for equipment (specifically, audio equipment or an air conditioner) installed in the passenger compartment of an automobile. With the technique disclosed in Patent Document 1, however, it is difficult to reduce the thickness of the touch switch, and accordingly, the disclosed touch switch is not suited for use in the control panel of, for example, audio equipment or air conditioner of an automobile. In the case of Patent Document 2, no problem arises if the touch detection electrode (transparent electrode 1a) used is one in number, but the disclosed technique fails to provide an effective solution in cases where a plurality of touch detection electrodes exist in the same panel, as in a control panel for an automobile device.
Specifically, in an EL panel with many independent light emitting regions (EL layer 1), the back electrode 1d generally has an area covering all light emitting regions so as to be used as a common electrode. However, where the EL panel is configured in the manner disclosed in Patent Document 2 such that the EL driver circuit is connected to the transparent electrode 1a and the back electrode 1d so as to operate based on the potential of the transparent electrode 1a serving also as the touch detection electrode, that is, the potential of the back electrode 1d is floating with respect to the ground potential, large electrostatic capacitance exists between the transparent electrode 1a, which constitutes an independent light emitting region, and the back electrode 1d. Consequently, when one of the multiple touch detection electrodes is touched by the operator, the touch is detected with respect to all switches at the same time (see FIG. 5).